1. Field of the Invention
The present invention relates to a selective or spot plating apparatus, hereinafter referred to as "selective plating apparatus", and more particularly to such an apparatus for plating onto an elongated strip of metal, hereinafter referred to as "strip", in a predetermined interval in the lengthwise direction thereof. The selective plating apparatus of this type can be advantageously employed for plating onto required portions of the segments of a strip, on which a number of the segments are continuously formed by stamping the strip in the lengthwise direction, such as a lead frame that is used for the manufacture of semiconductor devices, for example, IC's and LSI's.
2. Description of the Prior Art
When plating a strip such as a lead frame, particularly when being selectively plated, a pre-treatment process must first be performed to wash the surface of the strip in order to remove oils, oxide or contaminants. After the plating has been finished, furthermore, an after-treatment process must be performed to wash away the plating solution residing on the strip and then dry the strip. If the series of processes, i.e., pre-treatment process, selective plating process, and after-treatment process are to be continuously carried out while the strip is moved in the lengthwise direction, it is preferable that the strip be fed continuously in the pre-treatment station, intermittently in the selective plating station, and continuously in the after-treatment station. The reason for this is that, when selective plating, the strip must be stopped, because the plating is effected by masking the portions which are not be plated, supplying solution onto exposed preselected portions to be plated and supplying an electric current for a predetermined period of time. However, in the pre-treatment and after-treatment processes, continuously feeding the strip should be better than intermittently feeding it, in order that the surface of the strip be uniformly treated. Thus, in a line in which both continuous and intermittent feeding sections cooperatively exist together, the speed for feeding the strip must be in synchronism between the continuous and intermittent feeding sections, and it is desirable to apply a predetermined tension to the strip.
The strip is delivered from a discharge section, in which the strip is wound on a reel. If a strip having a small thickness such as a lead frame is pulled with a strong force, strip deformation, will occur. Usually, therefore, the strip should be slack for a required length between the strip discharge section and the pre-treatment station for plating. It is therefore necessary to bring the delivery speed at the strip discharge section into synchronism with the continuous feeding speed of the strip in the pre-treatment station.
With regard to the process of selectively plating in which the strip is intermittently fed, the plating can only be effected when the strip remains stationary. It is desirable, therefore, the number of portions to be selectively plated in a single plating operation should be as many as possible, or the time required for one cycle of plating should be as short as possible.